01 - In a large bowl, combine sliced apples, granulated sugar, brown sugar, flour, cinnamon, nutmeg, lemon juice, and a pinch of salt. Toss until apples are thoroughly coated with the spice mixture.
02 - Transfer the coated apple mixture to the bottom of a lightly greased crockpot (at least 4-quart capacity), spreading into an even layer.
03 - In a separate bowl, combine oats, flour, brown sugar, cinnamon, and salt. Add the cold, cubed butter and use your fingertips or a pastry cutter to work the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
04 - Sprinkle the crumb mixture evenly over the apples in the crockpot, ensuring complete coverage.
05 - Cover and cook on HIGH for 2 to 2.5 hours, or on LOW for 3.5 to 4 hours, until the apples are tender and the topping is golden and crisp.
06 - For a crunchier texture, remove the lid during the last 30 minutes of cooking to allow the topping to firm up.
07 - Serve warm, optionally topped with vanilla ice cream or whipped cream.